The present invention relates to a magnetic disk device, and particularly to a cramp mechanism for fixing a magnetic disk medium to a spindle hub of the magnetic disk device.
FIG. 2A is a front view illustrating an example of cramp mechanism of a conventional magnetic disk device, whereof a sectional view is illustrated in FIG. 2B.
Referring to FIG. 2B, a magnetic disk medium (hereafter simply called the disk medium) 13, consisting of one or more piled disks, is set on a spindle hub 12, in the example. A disk cramper 14 is fastened to the spindle hub 12 over the disk medium 13 with a plurality (six in the example) of cramp screws 15 ranged in a circle at even intervals around the hub center, which fixes the disk medium 13 by pressing the disk medium 13 with a cramp load through a cramping part D provided at a circular edge of the disk cramper 14.
However, there is a problem, in this type of cramp mechanism, that the cramp load pressing the disk medium 13 varies cyclically in circumferential direction, as illustrated in FIG. 5.
FIG. 5 is a graphic chart illustrating the cramp load pressing the disk medium 13 along the cramping part D, which is represented by a dashed line of FIG. 2A, of the disk cramper 14, wherein six peaks of the cramp load are found at points D1, D2, . . . , each representing a nearest point of the cramping part D from each of the plurality of cramp screws 15.
This variation of the cramp load causes warping or waving of the disk medium 13.
For resolving this problem, in some conventional magnetic disk device, cramp mechanism making use of heating shrinkage fit is applied. FIG. 3 is a sectional view illustrating an example of the cramp mechanism making use of the heating shrinkage fit.
Referring to FIG. 3, a disk cramper 24 is fastened to a spindle hub 22 by way of heating shrinkage fit at a fitting part E. The external diameter of the spindle hub 22 and the internal diameter of the disk cramper 24 are fabricated so as to make a close fit of the fitting part E. When they are assembled, the fitting part E of the disk cramper 24 is heated so as to make a clearance fit of the fitting part E.
The heated disk cramper 24 is inserted into and maintained on the spindle hub 22 with a pre-load added from outside evenly in circumferential direction at a cramping part F provided at a circular edge of the disk cramper 24. The pre-load is retained until the disk cramper 24 is made cool. Therefore, the disk cramper 24 is fastened to the spindle hub 22 at the fitting part E retaining the even pre-load, which works as an even cramp load for cramping the disk medium 23 with little warping or waving.
In the magnetic disk devices such as above described, the piled disk medium should be cramped by the cramp mechanism with a cramp load giving more than a certain pressure not to be shifted by an unexpected shock, and, at the same time, the disk medium should be cramped with minimum warping or waving for ensuring a stable posture of a magnetic head flying on the surface of the disk medium.
When there is warping or waving of the disk medium caused by uneven cramp load such as described in connection with the cramp mechanism of FIGS. 2A and 2B, the flying posture of the magnetic head becomes unstable, which degrades magnetoelectric performance of the magnetic head or may result even in a head crash. When tightening force of the cramp screws 15 is lightened for reducing variation of the cramping load, the disk medium 13 may be shifted with an unexpected shock, even if the problem of warping or waving is improved. Therefore, with the cramp mechanism of the first example of FIGS. 2A and 2B wherein the disk cramper 14 is fastened by the cramp screws 15, there is a limit in reduction of the flying clearance between the magnetic head and the disk medium, and consequently there is a limit in improvement of performance and reliability of the magnetic disk device.
As to the cramp mechanism making use of the heating shrinkage fit such as described in connection with FIG. 3, the disk cramper 24 is inserted to the spindle hub 22 being given with an even pre-load for cramping the disk medium 23 making use of size difference between the members made by thermal expansion, and tightened in the radial direction to the spindle hub after the disk cramper 24 is cooled. Therefore, an even cramp load is obtained and the problem of warping or waving because of the uneven cramp load made by cramp screws 15 of FIGS. 2A and 2B can be eliminated in the disk medium 23 of FIG. 3. However, there is a fear of bad influences remaining in the peripheral members caused by the heat treatment, on the other hand.
Furthermore, there is another problem that special equipment including a jug is needed exclusively for the heating shrinkage fit, which interferes the productivity in mass production of the magnetic disk device.